Troubleshooting Soft Start Drive

[jerjwillelec]

If this is 208 or 240 volts and you have three single phase heaters rated 240 volts just connect one between each output terminal (resulting in a delta configuration of the output loads). Total current at full voltage will be 240 (or 208) divided by 1.73.

If you have 120 volt heaters connect them in a wye configuration instead. One lead of each heater to an output lead of your starter and other leads all three tie together (the mid point of the wye) but do not ground this point. Each heater will see 120 volts if the input is 208, but will be higher if input is 240 - about 138 volts.

This is no different than connecting transformer or motor windings in same configurations.

Well, I don't have three 240 volt heaters laying around and I'm not in the greaest position to go out and buy them This is interesting and an approach I haven't taken before so just so I understand correctly... If I did have three 240 heaters I could put an equal amount of resistance on the drive by putting for example heater 1 on A and B, heater 2 on B and C, heater 3 on A and C?

 

[kwired]

Well, I don't have three 240 volt heaters laying around and I'm not in the greaest position to go out and buy them This is interesting and an approach I haven't taken before so just so I understand correctly... If I did have three 240 heaters I could put an equal amount of resistance on the drive by putting for example heater 1 on A and B, heater 2 on B and C, heater 3 on A and C?

You got it. That is a delta connected load (or three single phase loads connected to make a delta connected three phase load.

You could do same with three 120 volt heaters connected in wye, but if the source is 240 then the heaters will have about 138 volts applied instead of 120. Probably would not hurt anything if you limit run time though. I guess you could connect the supply neutral to the midpoint, but one of those phases is going to be 208 to ground if a high leg delta supply. If your supply is 208 volts no problem they will operate at 120.

 

[jerjwillelec]

You got it. That is a delta connected load (or three single phase loads connected to make a delta connected three phase load.

You could do same with three 120 volt heaters connected in wye, but if the source is 240 then the heaters will have about 138 volts applied instead of 120. Probably would not hurt anything if you limit run time though. I guess you could connect the supply neutral to the midpoint, but one of those phases is going to be 208 to ground if a high leg delta supply. If your supply is 208 volts no problem they will operate at 120.

Thanks..you're awesome! One other question to clarify...would this be in parallel with the motor or take the motor out of the equation for the test?

 

[jerjwillelec]

Btw...system voltage is 120/208 Y

 

[kwired]

Thanks..you're awesome! One other question to clarify...would this be in parallel with the motor or take the motor out of the equation for the test?

Either, the motor would just increase the load over what the heaters draw alone. Connecting (motor) should make the size of heater needed be smaller.

 

[jerjwillelec]

Either, the motor would just increase the load over what the heaters draw alone. Connecting should make the size of heater needed be smaller.

That's what I was thinking too. Thanks! I'll try to update...

 

[Besoeker]

9.9 times out of 10, this is the result of the load on the motor being too light.

Most soft starters on the market require that the current flowing through the current sensors inside of it be at least 20-30% of the rating of the unit (varies by mfr.). .

Most motors take that level of current even if unloaded.

 

[kwired]

Most motors take that level of current even if unloaded.

Most motors are loaded to that level, but that doesn't mean the soft starter is oversized for frequent starting or other derating reasons and maybe in that case an unloaded motor could have enough current level to be an issue.

 

[jerjwillelec]

So I took some resistance readings between the line and load of each phase. Phases A and B each show 115 k ohms while phase C shows 3.2 M ohms. Bad thyristor? Square D can rebuild it but I'm thinking it would be cheaper to replace with an ATS48.

 

[Besoeker]

So I took some resistance readings between the line and load of each phase. Phases A and B each show 115 k ohms while phase C shows 3.2 M ohms. Bad thyristor? Square D can rebuild it but I'm thinking it would be cheaper to replace with an ATS48.

In my experience thyristor failure is mostly short circuit,
Maybe there is other circuitry on those phases.

 

[Jraef]

You measure for a bad SCR in a soft starter by simply isolating it (no power anywhere, no motor windings connected) and measuring the resistance from line to load on each phase, L1 to T1, L2 to T2 etc. Anything less than 10K ohms is a shorted SCR in that pole. But all soft starters that I know of will tell you if they have a shorted SCR, they will have a similar detection circuit in the soft starter already. It can't tell if one is shorted while it is running, but it knows as soon as you turn it off. The test procedure is just telling you WHICH pole has the bad one.

Could be a bad gate circuit though, but again, most soft starters will annunciate a primary component failure like that. I had a lot of bad experiences with the firing boards and SCRs on the original Telemecanique ATS-23 soft starters, they were crap. The problem I think it was that they were not built for North America and the power supply boards did not like the 60Hz power, which were powering the pulse transformers in the SCR gate circuit and saturating them, which caused them to fail or caused the gate junctions in the SCRs to fail. On one project I had 8 failures on 6 installed soft starters within the first 6 months, had to yank them all out and replace them at the owner's insistence. I would have thought Schneider would have fixed that by now though.